There are not that many things that would take advantage of it as a PhysX card. Like BSD suggested, I would try to sell it and get a different card that you could SLI with. If that isn't an option, and your motherboard/PSU can support adding the card in, you might as well.
You can only run cards in SLI/crossfire if they're the exact same. Only way to guarantee this is if you buy both at the same time. If they're completely different models, don't waste your time. I'd do what FallenSniper stated, sell both cards and buy a better card that will out perform both of those cards. Hint: GTX285 will out perform both of those cards hands down.